Calcium sulfate alpha hemihydrate is a product which can be obtained from the sludges resulting from the desulfurization of flue gases utilizing a wet scrubbing process in which calcium oxide or calcium carbonate is added to the scrubbing solution.
This product is, with increasing environmental concern with respect to acid rain and pollution of the environment by the flue gases, produced in increasing quantities and an effective use for the product eliminates, of course, problems which may result from disposal of desulfurization residue.
On the other hand, it is known to use the calcium sulfate alpha hemihydrate in hydraulic binders. Indeed, in the literature summary (Bauakademie der DDR, Bauinformation, Literaturzusammenstellung Nr. 7/2483, Titel 4., Gordasevski inter alia), calcium sulfate alpha hemihydrate and Portland cement can be mixed and pozzolan, depending upon the calcium oxide content, can be added, thereby making use of the calcium sulfate alpha hemihydrate as a component of the resulting hydraulic binder contributing a fast-setting action to the binder. Tests have shown, however, that the water resistance of a hardened mixture of this latter three-component binder is not sufficiently reproducible and may not be satisfactory for many purposes.
In the present description, we may refer to "fast hardening strength" to mean the compressive strength which the hydraulic binder, upon make-up with water, generates during its initial setting period. The "water resistance" to which we refer is, of course, the resistance of the set product to water as evidenced, for example, by its low water take-up. The binder generally is provided in finely milled form and may be made-up with a sufficient quantity of water to effect the setting.
In the past, when calcium sulfate alpha hemihydrate has been used as a significant component of a fast-setting water resistant binder, i.e. a binder which, when made-up with water, yields a fast-setting composition which is resistant, upon setting, to water, it has been the practice to add hydrophobic or hydrophobizing agents to the composition. Paraffin is one such hydrophobizing agent. The product thus obtained has good water resistance or, stated otherwise, can have the desired degree of water resistance depending upon the quantity or proportion of the paraffin used, but the effect is only a short-term effect and the hydrophobizing agent is generally too expensive for use in a large-scale application.